Symmetrical output neurostimulation device

ABSTRACT

A method and system of providing therapy to a patient using electrodes implanted adjacent tissue. The method comprises regulating a first voltage at an anode of the electrodes relative to the tissue, regulating a second voltage at a cathode of the electrodes relative to the tissue, and conveying electrical stimulation energy between the anode at the first voltage and the cathode at the second voltage, thereby stimulating the neural tissue. The system comprises a grounding electrode configured for being placed in contact with the tissue, electrical terminals configured for being respectively coupled to the electrodes, a first regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, a second regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, and control circuitry configured for controlling the regulators to convey electrical stimulation energy between the anode and cathode.

RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119 to U.S.provisional patent application Ser. No. 61/220,131, filed Jun. 24, 2009.The foregoing application is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations PeripheralNerve Stimulation (PNS) systems have demonstrated efficacy in thetreatment of chronic pain syndromes and incontinence, and a number ofadditional applications are currently under investigation. Furthermore,Functional Electrical Stimulation (FES) systems such as the Freehandsystem by NeuroControl (Cleveland, Ohio) have been applied to restoresome functionality to paralyzed extremities in spinal cord injurypatients.

Each of these implantable neurostimulation systems typically includes anelectrode lead implanted at the desired stimulation site and animplantable pulse generator (IPG) implanted remotely from thestimulation site, but coupled either directly to the electrode lead orindirectly to the electrode lead via a lead extension. Thus, electricalpulses can be delivered from the neurostimulator to the stimulationelectrode(s) to stimulate or activate a volume of tissue in accordancewith a set of stimulation parameters and provide the desired efficacioustherapy to the patient. A typical stimulation parameter set may includethe electrodes that are sourcing (anodes) or returning (cathodes) thestimulation current at any given time, as well as the amplitude,duration, rate, and burst rate of the stimulation pulses.

The neurostimulation system may further comprise a handheld remotecontrol (RC) to remotely instruct the neurostimulator to generateelectrical stimulation pulses in accordance with selected stimulationparameters. The RC may, itself, be programmed by a technician attendingthe patient, for example, by using a Clinician's Programmer (CP), whichtypically includes a general purpose computer, such as a laptop, with aprogramming software package installed thereon.

Electrical stimulation energy may be delivered from the neurostimulatorto the electrodes using one or more current-controlled sources forproviding stimulation pulses of a specified and known current (i.e.,current regulated output pulses), or one or more voltage-controlledsources for providing stimulation pulses of a specified and knownvoltage (i.e., voltage regulated output pulses). The circuitry of theneurostimulator may also include voltage converters, power regulators,output coupling capacitors, and other elements as needed to produceconstant voltage or constant current stimulus pulses. Conventionalbattery-operated neurostimulators typically apply stimulation pulses tothe tissue that are referenced to an internal circuit voltage in theneurostimulator, with a relatively low impedance connection beinglocated between one or more stimulation electrodes and internalcircuitry. This relatively low impedance effectively clamps the voltageon these stimulation electrodes to the internal circuit voltage.

For example, a voltage source can be coupled between the internalcircuitry and an anode to create a cathode clamped voltage regulatedcircuit (FIG. 1 a), a current source can be coupled between the internalcircuitry and an anode to create a cathode clamped current regulatedcircuit (FIG. 1 b), a voltage source can be coupled between the internalcircuitry and a cathode to create an anode clamped voltage regulatedcircuit (FIG. 1 c), and a current source can be coupled between theinternal circuitry and a cathode to create an anode clamped currentregulated circuit (FIG. 1 d). It can be appreciated that the referencevoltage will be at the cathodes for the topologies illustrated in FIGS.1 a and 1 b and will be at the anodes for the topologies illustrated inFIGS. 1 c and 1 d.

Because the voltage at the unregulated side of the electrode will beclamped to the voltage of the internal circuitry, and because thestimulation output circuitry may be unbalanced in that some componentsin the circuitry (coupling capacitors, protection circuits, etc.) may bepresent on the cathode side of the circuit but not the anode side of thecircuit, or vice versa, the output stimulation circuitry between thecathode and the anode will be asymmetrical, such that the cathode andthe anode will be asymmetrically referenced to the internal circuit. Forexample, a shift in voltage in the output stimulation circuit results inasymmetrical voltage shifts between the anodes and cathodes.

In particular, the voltage of the common mode signal (i.e., the averageof the anode voltage shift and cathode voltage shift relative to thereference voltage) will be equal to or greater than the differentialvoltage between the cathode and anode. For example, as shown in FIG. 2a, when the cathode voltage is at the internal reference voltage, thecommon mode signal is equal to one-half the differential voltage betweenthe cathode and anode. As shown in FIG. 2 b, when the cathode voltage isabove the internal reference voltage, the voltage of the common modesignal is greater than one-half the differential voltage between thecathode and anode. As shown in FIG. 2 c, when the cathode voltage isbelow the internal reference voltage, the voltage of the common modesignal is likewise greater than one-half the differential voltagebetween the cathode and anode. The asymmetry between anodes and cathodesin the output stimulation circuitry may be associated with undesiredside effects during stimulation that lead to reduced patient comfort. Inparticular, parasitic coupling of the common mode signal to theimplantable device can give rise to an additional stimulation signalthat is superimposed on the differential stimulation signal. Even if thecommon mode signal is subthreshold by itself, it can modulate thedifferential stimulation signal, causing unwanted activation of neuraltissue.

In addition to the problem of asymmetry in the output stimulationcircuit, referencing the voltage at the cathodes and anodes to aninternal circuit may require excessive voltage levels at the cathodesand anodes in order to maintain the desired voltage potentialtherebetween. For example, if the desired voltage potential between acathode and an anode is 5V, and if the internal voltage is 20V, thevoltage at the anode would have to be 25V and the voltage at the cathodewould have to be 20V. The increased voltage at the electrodes willincrease the voltage relative to the tissue, which may cause problemssuch as unwanted stimulation and even electro-chemical reactionsresulting in corrosion of the electrodes.

There, thus, remains a need for an improved method and system forconveying stimulation to tissue in a controlled manner.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a method ofproviding therapy to a patient using an array of electrodes implantedadjacent neural tissue (e.g., spinal cord tissue) of the patient isprovided. The method comprises regulating a first voltage at an anode ofthe electrodes relative to the neural tissue, regulating a secondvoltage at a cathode of the electrodes relative to the tissue, andconveying electrical stimulation energy between the anode at the firstvoltage and the cathode at the second voltage, thereby stimulating theneural tissue. In one method, the voltages on the anode and cathode areregulated in a balanced fashion, such that an average shift in voltageon the anode and cathode relative to the neural tissue is equal to orless than one half a differential voltage between the anode and cathode.Optimally, the voltage shifts at the anode and cathode relative to theneural tissue may be equal in magnitude, but opposite in polarity (i.e.,anode voltage shifts up and cathode voltage shifts down by the amount).An optional method comprises regulating a first current flowing throughthe anode, and regulating a second current flowing through the cathode.Furthermore, the values for the first current and the second currentnecessary to achieve the first and second voltages may be computed.

In accordance with a second aspect of the present inventions, aneurostimulation system is provided. The neurostimulation systemcomprises a grounding electrode configured for being placed in contactwith neural tissue, and a plurality of electrical terminals configuredfor being respectively coupled to an array of electrodes. Theneurostimulation system further comprises a first regulator configuredfor being electrically coupled between an anode of the electrodes andthe grounding electrode, a second regulator configured for beingelectrically coupled between an anode of the electrodes and thegrounding electrode, and control circuitry configured for controllingthe first and second regulators to convey electrical stimulation energybetween the anode and the cathode.

In one embodiment, each of the first and second regulators comprises avoltage source. In another embodiment, each of the first and secondregulators comprises a current source. In the latter case, the controlcircuitry is configured for controlling the current sources to outputthe same current value and/or the control circuitry may be furtherconfigured for determining values for the first current and the secondcurrent necessary to achieve the first and second voltages. In anotherembodiment, the control circuitry may be configured for controlling theregulators such that an average shift in voltage on the anode andcathode relative to the neural tissue is equal to or less than one halfa differential voltage between the anode and cathode. Optimally, thevoltage shifts at the anode and cathode relative to the neural tissuemay be equal in magnitude, but opposite in polarity (i.e., anode voltageshifts up and cathode voltage shifts down by the amount). Theneurostimulation system may comprise a housing containing the pluralityof electrical terminals, first and second voltage regulators, andcontrol circuitry.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIGS. 1 a-1 d are prior art circuit diagrams of different prior arttissue stimulation topologies;

FIGS. 2 a-2 c are prior art diagrams of a cathode voltage and an anodevoltage generated by the tissue stimulation topologies of FIGS. 1 a-1 d;

FIG. 3 is plan view of one embodiment of a spinal cord stimulation (SCS)system arranged in accordance with the present inventions;

FIG. 4 is a plan view of the SCS system of FIG. 3 in use with a patient;

FIG. 5 is a profile view of an implantable pulse generator (IPG) used inthe SCS system of FIG. 3;

FIG. 6 is a block diagram of the internal components of the IPG of FIG.5;

FIGS. 7 a and 7 b are circuit diagrams of two tissue stimulationtopologies used by the SCS system of FIG. 3;

FIGS. 8 a and 8 b are circuit diagrams of two alternative tissuestimulation topologies used by the SCS system of FIG. 3;

FIGS. 9 a and 9 b are circuit diagrams of two alternative tissuestimulation topologies used by the SCS system of FIG. 3; and

FIG. 10 is a diagram of a cathode voltage and an anode voltage generatedby the tissue stimulation topologies of FIGS. 1 a-1 d.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a spinal cord stimulation (SCS)system. However, it is to be understood that the while the inventionlends itself well to applications in SCS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, peripheral nerve stimulator, microstimulator,or in any other neural stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 3, an exemplary spinal cord stimulation (SCS)system 10 generally includes one or more (in this case, two) implantablestimulation leads 12, a pulse generating device in the form of animplantable pulse generator (IPG) 14, an external control device in theform of a remote controller RC 16, a clinician's programmer (CP) 18, anexternal trial stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the stimulation leads 12, which carry a plurality ofelectrodes 26 arranged in an array. In the illustrated embodiment, thestimulation leads 12 are percutaneous leads, and to this end, theelectrodes 26 are arranged in-line along the stimulation leads 12. Inalternative embodiments, the electrodes 26 may be arranged in atwo-dimensional pattern on a single paddle lead. As will be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the stimulation leads 12. The ETS20, which has similar pulse generation circuitry as that of the IPG 14,also delivers electrical stimulation energy to the electrode array 26 inaccordance with a set of stimulation parameters. The major differencebetween the ETS 20 and the IPG 14 is that the ETS 20 is anon-implantable device that is used on a trial basis after thestimulation leads 12 have been implanted and prior to implantation ofthe IPG 14, to test the responsiveness of the stimulation that is to beprovided. Further details of an exemplary ETS are described in U.S. Pat.No. 6,895,280, which is expressly incorporated herein by reference.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown). The clinician detailed stimulation parameters provided by the CP18 are also used to program the RC 16, so that the stimulationparameters can be subsequently modified by operation of the RC 16 in astand-alone mode (i.e., without the assistance of the CP 18). Theexternal charger 22 is a portable device used to transcutaneously chargethe IPG 14 via an inductive link 38. Once the IPG 14 has beenprogrammed, and its power source has been charged by the externalcharger 22 or otherwise replenished, the IPG 14 may function asprogrammed without the RC 16 or CP 18 being present.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

As shown in FIG. 4, the electrode leads 12 are implanted within thespinal column 42 of a patient 40. The preferred placement of theelectrode leads 12 is adjacent, i.e., resting upon near, or upon thedura, adjacent to the spinal cord area to be stimulated. Due to the lackof space near the location where the electrode leads 12 exit the spinalcolumn 42, the IPG 14 is generally implanted in a surgically-made pocketeither in the abdomen or above the buttocks. The IPG 14 may, of course,also be implanted in other locations of the patient's body. The leadextension 24 facilitates locating the IPG 14 away from the exit point ofthe electrode leads 12. As there shown, the CP 18 communicates with theIPG 14 via the RC 16.

Referring now to FIG. 5, the external features of the stimulation leads12 and the IPG 14 will be briefly described. One of the stimulationleads 12 has eight electrodes 26 (labeled E1-E8), and the otherstimulation lead 12 has eight electrodes 26 (labeled E9-E16). The actualnumber and shape of leads and electrodes will, of course, vary accordingto the intended application. The IPG 14 comprises an outer case 50 forhousing the electronic and other components (described in further detailbelow), and a connector 52 to which the proximal ends of the stimulationleads 12 mate in a manner that electrically couples the electrodes 26 tothe internal electronics (described in further detail below) within theouter case 50. The outer case 50 is composed of an electricallyconductive, biocompatible material, such as titanium, and forms ahermetically sealed compartment wherein the internal electronics areprotected from the body tissue and fluids. In some cases, the outer case50 may serve as an electrode.

As briefly discussed above, the IPG 14 includes battery and pulsegeneration circuitry that delivers the electrical stimulation energy inthe form of a pulsed electrical waveform to the electrode array 26 inaccordance with a set of stimulation parameters programmed into the IPG14. Such stimulation parameters may comprise electrode combinations,which define the electrodes that are activated as anodes (positive),cathodes (negative), and turned off (zero), percentage of stimulationenergy assigned to each electrode (fractionalized electrodeconfigurations), and electrical pulse parameters, which define the pulseamplitude (measured in milliamps or volts depending on whether the IPG14 supplies constant current or constant voltage to the electrode array26), pulse width (measured in microseconds), and pulse rate (measured inpulses per second), pulse shape, and burst rate (measured as thestimulation on duration per unit time).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case 50. Simulation energy maybe transmitted to the tissue in a monopolar or multipolar (e.g.,bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when aselected one of the lead electrodes 26 is activated along with the case50 of the IPG 14, so that stimulation energy is transmitted between theselected electrode 26 and case 50. Bipolar stimulation occurs when twoof the lead electrodes 26 are activated as anode and cathode, so thatstimulation energy is transmitted between the selected electrodes 26.For example, electrode E3 on the first lead 12 may be activated as ananode at the same time that electrode E11 on the second lead 12 isactivated as a cathode. Tripolar stimulation occurs when three of thelead electrodes 26 are activated, two as anodes and the remaining one asa cathode, or two as cathodes and the remaining one as an anode. Forexample, electrodes E4 and E5 on the first lead 12 may be activated asanodes at the same time that electrode E12 on the second lead 12 isactivated as a cathode.

The stimulation energy may be delivered between electrodes as monophasicelectrical energy or multiphasic electrical energy. Monophasicelectrical energy includes a series of pulses that are either allpositive (anodic) or all negative (cathodic). Multiphasic electricalenergy includes a series of pulses that alternate between positive andnegative. For example, multiphasic electrical energy may include aseries of biphasic pulses, with each biphasic pulse including a cathodic(negative) stimulation pulse and an anodic (positive) recharge pulsethat is generated after the stimulation pulse to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is conveyed through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the recharge pulse).

Turning next to FIG. 6, the main internal components of the IPG 14 willnow be described. The IPG 14 includes analog output circuitry 60configured for generating electrical stimulation energy in accordancewith a defined pulsed waveform having a specified pulse amplitude, pulserate, pulse width, pulse shape, and burst rate under control of controllogic 62 over data bus 64. Control of the pulse rate and pulse width ofthe electrical waveform is facilitated by timer logic circuitry 66,which may have a suitable resolution, e.g., 10 μs. The electricalstimulation energy generated by the output analog circuitry 60 is outputvia capacitors C1-C16 to electrical terminals 68 corresponding to theelectrodes 26.

The analog output circuitry 60 may either comprise independentlycontrolled current sources for providing electrical stimulation energyof a specified and known amperage to or from the electrical terminals68, or independently controlled voltage sources for providing electricalstimulation energy of a specified and known voltage at the electricalterminals 68 or to multiplexed current or voltage sources that are thenconnected to the electrical terminals 68. The operation of the analogoutput circuitry 60, including alternative embodiments of suitableoutput circuitry for performing the same function of generatingstimulation pulses of a prescribed amplitude and width, is describedmore fully in U.S. Pat. Nos. 6,516,227 and 6,993,384, which areexpressly incorporated herein by reference.

Significantly, as will be described in further detail below, the analogoutput circuitry 60 presents symmetrical outputs to both the anodes andcathodes that will not be subject to the differential voltage shifts inthe circuitry discussed in the background. Furthermore, the analogoutput circuitry 60 references the voltages at the anodes and cathodesto the tissue rather than a voltage internal to the IPG 14.

The IPG 14 further comprises monitoring circuitry 70 for monitoring thestatus of various nodes or other points 72 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. Themonitoring circuitry 70 is also configured for measuring electricalparameter data (e.g., electrode impedance and/or electrode fieldpotential). The IPG 14 further comprises processing circuitry in theform of a microcontroller (μC) 74 that controls the control logic 62over data bus 76, and obtains status data from the monitoring circuitry70 via data bus 78. The IPG 14 further comprises memory 80 andoscillator and clock circuit 82 coupled to the microcontroller 74. Themicrocontroller 74, in combination with the memory 80 and oscillator andclock circuit 82, thus comprise a microprocessor system that carries outa program function in accordance with a suitable program stored in thememory 80. Alternatively, for some applications, the function providedby the microprocessor system may be carried out by a suitable statemachine.

Thus, the microcontroller 74 generates the necessary control and statussignals, which allow the microcontroller 74 to control the operation ofthe IPG 14 in accordance with a selected operating program andstimulation parameters. In controlling the operation of the IPG 14, themicrocontroller 74 is able to individually generate stimulus pulses andelectrical background energy at the electrical terminals 68 using theanalog output circuitry 60, in combination with the control logic 62 andtimer logic circuitry 66, thereby allowing each electrical terminal 68(and thus, each electrode 26) to be paired or grouped with otherelectrical terminals 68 (and thus, other electrodes 26), including themonopolar case electrode, to control the polarity, amplitude, rate,pulse width, pulse shape, burst rate, and channel through which thecurrent stimulus pulses and associated electrical background energy areprovided. The microcontroller 74 facilitates the storage of electricalparameter data measured by the monitoring circuitry 70 within memory 80.

The IPG 14 further comprises a receiving coil 84 for receivingprogramming data (e.g., the operating program and/or stimulationparameters) from the external programmer (i.e., the RC 16 or CP 18) inan appropriate modulated carrier signal, and charging, and circuitry 86for demodulating the carrier signal it receives through the receivingcoil 84 to recover the programming data, which programming data is thenstored within the memory 80, or within other memory elements (not shown)distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 88 and atransmission coil 90 for sending informational data to the externalprogrammer. The back telemetry features of the IPG 14 also allow itsstatus to be checked. For example, when the CP 18 initiates aprogramming session with the IPG 14, the capacity of the battery istelemetered, so that the CP 18 can calculate the estimated time torecharge. Any changes made to the current stimulus parameters areconfirmed through back telemetry, thereby assuring that such changeshave been correctly received and implemented within the implant system.Moreover, upon interrogation by the CP 18, all programmable settingsstored within the IPG 14 may be uploaded to the CP 18.

The IPG 14 further comprises a rechargeable power source 92 and powercircuits 94 for providing the operating power to the IPG 14. Therechargeable power source 92 may, e.g., comprise a lithium-ion orlithium-ion polymer battery or other form of rechargeable power. Therechargeable source 92 provides an unregulated voltage to the powercircuits 94. The power circuits 94, in turn, generate the variousvoltages 96, some of which are regulated and some of which are not, asneeded by the various circuits located within the IPG 14. Therechargeable power source 92 is recharged using rectified AC power (orDC power converted from AC power through other means, e.g., efficientAC-to-DC converter circuits, also known as “inverter circuits”) receivedby the receiving coil 84.

To recharge the power source 92, the external charger 22 (shown in FIG.3), which generates the AC magnetic field, is placed against, orotherwise adjacent, to the patient's skin over the implanted IPG 14. TheAC magnetic field emitted by the external charger induces AC currents inthe receiving coil 84. The charging and forward telemetry circuitry 86rectifies the AC current to produce DC current, which is used to chargethe power source 92. While the receiving coil 84 is described as beingused for both wirelessly receiving communications (e.g., programming andcontrol data) and charging energy from the external device, it should beappreciated that the receiving coil 84 can be arranged as a dedicatedcharging coil, while another coil, such as the coil 90, can be used forbi-directional telemetry.

Additional details concerning the above-described and other IPGs may befound in U.S. Pat. No. 6,516,227, U.S. Patent Publication No.2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled“Low Power Loss Current Digital-to-Analog Converter Used in anImplantable Pulse Generator,” which are expressly incorporated herein byreference.

It should be noted that rather than an IPG, the SCS system 10 mayalternatively utilize an implantable receiver-stimulator (not shown)connected to the stimulation leads 12. In this case, the power source,e.g., a battery, for powering the implanted receiver, as well as controlcircuitry to command the receiver-stimulator, will be contained in anexternal controller inductively coupled to the receiver-stimulator viaan electromagnetic link. Data/power signals are transcutaneously coupledfrom a cable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation energy and background energy inaccordance with the control signals.

As briefly discussed above, the analog output circuitry 60 presentssymmetrical outputs to both the anodes and cathodes. For example, withreference to FIG. 7 a, a first voltage source 102 a is coupled to ananode 100 a, and a second voltage source 102 b is coupled to a cathode100 b. This is in contrast to the single-ended voltage regulated circuitillustrated in FIG. 1 a. Thus, because there is a voltage source at boththe anode 100 a and the cathode 100 b, voltage shifts within the analogoutput circuitry 60 will not be conducted to the anode 100 a and cathode100 b differentially. Alternatively, with reference to FIG. 7 b, a firstcurrent source 104 a is coupled to the anode 100 a, and a second currentsource 104 b is coupled to the cathode 100 b. This is in contrast to thesingle-ended current regulated circuit illustrated in FIG. 1 b. In thiscase, the current sources present a high impedance to the respectiveanode 100 a and cathode 100 b, thereby isolating the anode 100 a andcathode 100 b from voltage shifts within the analog output circuitry 60.

As also briefly discussed above, the analog output circuitry 60references the voltages at the anodes and cathodes to the tissue ratherthan a voltage internal to the IPG 14. To this end, the IPG 14 isprovided with a grounding electrode 106 configured for being placed incontract with tissue. For example, the grounding electrode 106 may belocated on the case 50 or may be the case 50 itself. In the illustratedembodiment, the analog output circuitry 60 regulates the voltages at theanodes and cathodes, such that the common mode signal (i.e., the averageof the anode voltage shift and cathode voltage shift relative to thereference voltage (in this case, the grounding electrode 106)) will beequal to or less than the differential voltage between the cathodes andanodes, as illustrated in FIG. 10.

With reference back to FIG. 7 a, the first voltage source 102 a iselectrically coupled between the anode 100 a and the grounding electrode106, and the second voltage source 102 b is electrically coupled betweenthe cathode 100 b and the grounding electrode 106. As a result, thevoltages at the respective anode 100 a and cathode 100 b relative to thetissue may be controlled, so that large voltages are not applied to thetissue. The voltage values respectively output by the first and secondvoltage sources 102 a, 102 b can be set to be equal in order to minimizethe maximum voltage seen by the tissue. For example, if the desiredvoltage potential between the anode 100 a and the cathode 100 b is 5V,the first voltage source 102 a can be set to output a voltage of 2.5Vrelative to the grounding electrode 106 (and thus, the tissue), and thesecond voltage source 102 b can be set to output a voltage of −2.5Vrelative to the grounding electrode 106 (and thus, the tissue).Essentially, in this case, the voltage of the common mode signal wouldbe zero. Notably, the internal reference voltage of the analog outputcircuitry 60 is irrelevant, since the voltage sources 102 a, 102 b arenot referenced to this internal voltage.

With reference to FIG. 7 b, the first current source 104 a iselectrically coupled between the anode 100 a and the grounding electrode106, and the second voltage source 102 b is electrically coupled betweenthe cathode 100 b and the grounding electrode 106. Thus, the electricalcurrent flowing through each of the anode 100 a and the cathode 100 bcan be controlled. In this case, where there the anode 100 a and cathode100 b are the only active electrodes, the absolute value of theelectrical current magnitude flowing through the anode 100 a will beessentially equal to the electrical current magnitude flowing throughthe cathode 100 b; however, the electrical currents flowing through theanode 100 a and cathode 100 b will be oppositely polarized. For example,the current output by the first current source 104 a may be set at 2.5mA, while the current output by the second current source 104 b may beset at −2.5 mA. Essentially, in this case, the voltage of the commonmode signal would be zero assuming that the tissue impedances on thecathodes and anodes are equal.

Although the current sources 104 a, 104 b regulate the current flowingthrough the anode 100 a and cathode 100 b, the voltages at therespective anode 100 a and cathode 100 b relative to the tissue maystill be controlled, so that large voltages are not applied to thetissue. In particular, the currents required to be output by therespective current sources 100 a, 100 b to achieve the voltagedistribution desired at the respective anode 100 a and cathode 100 brelative to the tissue can be computed in a conventional manner.

Although each of the voltage sources 102 a, 102 b and current sources104 a, 104 b in the topologies illustrated in FIGS. 7 a and 7 b arecoupled to only a single electrode, it should be appreciated that eachof these sources can be coupled to multiple electrodes (either a groupof anodes 100 a or a group of cathodes 100 b), as illustrated in FIGS. 8a and 8 b. Furthermore, multiple sources of the same type can berespectively connected to multiple electrodes at the same time. Forexample, two voltage sources 102 a or two current sources 104 a can berespectively connected to two anodes 100 a at the same time, or twovoltage sources 102 b or two current sources 104 b can be respectivelyconnected to two cathodes 100 at the same time, as illustrated in FIGS.9 a and 9 b. Thus, this concept can be applied to a multiplicity ofanodes and a multiplicity of cathodes where the positive shifts involtage on the anode and negative shifts in voltage on the cathodes aresuch that the average shift is zero or at least less than one half ofthe maximum differential voltage between any anode and cathode pairduring the stimulation pulse.

It can be appreciated from the foregoing that the voltage or voltages atthe anode or anodes 100 a relative to the tissue can be regulated, andthe voltage or voltages at the cathode or cathodes 100 b can beregulated, while the electrical stimulation energy is conveyed betweenthe anode or anodes 100 a and the cathode or cathodes 100 b. If any ofthe topologies illustrated in FIGS. 7 b, 8 b, or 9 b or used, thecurrents flowing through the anode or anodes 100 a and the cathode orcathodes 100 b can be regulated

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. A method of providing therapy to a patient using an array ofelectrodes implanted adjacent neural tissue of the patient, the methodcomprising: regulating a first voltage at an anode of the electrodesrelative to the neural tissue; regulating a second voltage at a cathodeof the electrodes relative to the neural tissue; and conveyingelectrical stimulation energy between the anode at the first voltage andthe cathode at the second voltage, thereby stimulating the neuraltissue.
 2. The method of claim 1, further comprising: regulating a firstcurrent flowing through the anode; and regulating a second currentflowing through the cathode.
 3. The method of claim 2, furthercomprising determining values for the first current and the secondcurrent necessary to achieve the first and second voltages.
 4. Themethod of claim 1, wherein the first voltage on the anode and the secondvoltage on the cathode are regulated, such that an average shift involtage on the anode and cathode relative to the neural tissue is equalto or less than one half a differential voltage between the anode andcathode.
 5. The method of claim 4, wherein the first voltage on theanode shifts upward relative to the neural tissue the same amount as thesecond voltage on the cathode shifts downward relative to the neuraltissue.
 6. The method of claim 1, wherein the neural tissue is spinalcord tissue.
 7. The method of claim 1, further comprising implanting thearray of electrodes within the patient.
 8. A neurostimulation system,comprising: a grounding electrode configured for being placed in contactwith neural tissue; a plurality of electrical terminals configured forbeing respectively coupled to an array of electrodes; a first regulatorconfigured for being electrically coupled between an anode of theelectrodes and the grounding electrode; a second regulator configuredfor being electrically coupled between an anode of the electrodes andthe grounding electrode; and control circuitry configured forcontrolling the first and second regulators to convey electricalstimulation energy between the anode and the cathode.
 9. Theneurostimulation system of claim 8, wherein each of the first and secondregulators comprises a voltage source.
 10. The neurostimulation systemof claim 8, wherein each of the first and second regulators comprises acurrent source.
 11. The neurostimulation system of claim 10, wherein thecontrol circuitry is further configured for determining values for thefirst current and the second current necessary to achieve the first andsecond voltages.
 12. The neurostimulation system of claim 8, wherein thecontrol circuitry is configured for controlling the regulators such thatan average shift in voltage on the anode and cathode relative to thegrounding electrode is equal to or less than one half a differentialvoltage between the anode and cathode.
 13. The neurostimulation systemof claim 12, wherein the control circuitry is configured for controllingthe regulators to shift the voltage on the anode upward relative to thegrounding electrode the same amount as the voltage on the cathode shiftsdownward relative to the grounding electrode.
 14. The neurostimulationsystem of claim 8, further comprising the array of electrodes.
 15. Theneurostimulation system of claim 8, further comprising a housingcontaining the plurality of electrical terminals, first and secondvoltage regulators, and control circuitry.